Errors in quantitative T1rho imaging and the correction methods

Abstract

The spin-lattice relaxation time constant in rotating frame (T1rho) is useful for assessment of the properties of macromolecular environment inside tissue. Quantification of T1rho is found promising in various clinical applications. However, T1rho imaging is prone to image artifacts and quantification errors, which remains one of the greatest challenges to adopt this technique in routine clinical practice. The conventional continuous wave spin-lock is susceptible to B1 radiofrequency (RF) and B0 field inhomogeneity, which appears as banding artifacts in acquired images. A number of methods have been reported to modify T1rho prep RF pulse cluster to mitigate this effect. Adiabatic RF pulse can also be used for spin-lock with insensitivity to both B1 RF and B0 field inhomogeneity. Another source of quantification error in T1rho imaging is signal evolution during imaging data acquisition. Care is needed to affirm such error does not take place when specific pulse sequence is used for imaging data acquisition. Another source of T1rho quantification error is insufficient signal-to-noise ratio (SNR), which is common among various quantitative imaging approaches. Measurement of T1rho within an ROI can mitigate this issue, but at the cost of reduced resolution. Noise-corrected methods are reported to address this issue in pixel-wise quantification. For certain tissue type, T1rho quantification can be confounded by magic angle effect and the presence of multiple tissue components. Review of these confounding factors from inherent tissue properties is not included in this article.

Keywords: MRI; Quantitative imaging; T1rho; artifacts correction.

Figure 1

A schematic diagram of conventional T1rho prep using continuous wave spin-lock approach. A 90-degree RF pulse (tip-down RF) flips magnetization into transverse plane. A RF pulse is then applied in parallel to magnetization to “spin-lock” the magnetization. After time of-spin-lock (TSL), the magnetization is flipped back to longitudinal direction by another 90-degree RF pulse (tip-up RF). A crusher is then used to dephase residual signal in transverse plane. 2D or 3D data acquisition methods are used to collected data after T1rho prep. RF, radiofrequency.

Figure 2

RF pulse clusters used to compensate B₁ RF and B₀ field inhomogeneity for continues wave spin-lock. (A) Rotary echo approach for B₁ RF inhomogeneity compensation. The phase of the second half of spin-lock is reversed; (B) the composite RF approach for B₀ field inhomogeneity compensation. A 135-degree RF pulse with same phase as spin-lock RF pulse is played out before and after spin-lock. The amplitude ratio between the composite RF pulse and spin-lock RF pulse is (); (C) simultaneous compensation of B₁ RF and B₀ field inhomogeneity. A 180-degree refocusing RF pulse is inserted in the middle of rotary echo and the phase of the tip-down RF is reversed compared to conventional method; (D) combination of the composite RF pulse and a phase cycling method for simultaneous B₁ RF and B₀ field inhomogeneity compensation. Two data sets are acquired with opposite phase of the tip-up RF pulse and subtracted from each other to yield final T1rho-preped images. No particular requirement of the amplitude ratio between the composite RF pulse and the spin-lock RF pulse. RF, radiofrequency.